The present invention relates generally to an exposure method, and more particularly to a measurement of a change in shape of a mask (or a reticle) on which a pattern is formed that is transferred to a plate via a projection optical system. The exposure method and apparatus of this invention is suitable for an exposure method and apparatus that corrects an aberration in a projection optical system and a focus position based on information about mask's flatness.
In manufacturing a semiconductor device and the like using a photo-lithography process, a projection exposure apparatus has been used that transfers a pattern of a mask to a target. Such exposure apparatus is required to precisely transfer a mask pattern onto the target, and should use an aberration reduced projection optical system for exposure at the best focus position. Further, due to the recent demand for a higher resolution, the projection optical system's numerical aperture (NA) increases. Thus, the depth of focus consequently lowers, and driving correction based on a mask's flatness becomes necessary.
One means to measure the mask's flatness is to optically detect a surface position is known (e.g., see Japanese Patent Application, Publication No. 9-180989 and PCT International Patent Application No. 2/43123 pamphlet). If such a means is installed on an exposure apparatus to measure the mask's flatness prior to exposure, the result can be fed back to correct a driving amount for driving a driving system (a wafer stage, a mask stage, etc.), and field curvature of a projector lens, which will assure highly precise exposure.
The focus condition and field curvature corrected based on an optically detected result may contain aberrations, and need corrections based on an actual exposure result. As an example of such a correction method, there is a phase-shift focus monitor (PSFM) technique available for use (See the Internet URL: http://www.benchmarktech.com/PSFM.htm.) In addition, a phase grating focus monitor (PGFM) is available (see “New phase shift gratings for measuring aberrations” by H. Nomura, SPIE. Vol. 4346 (2001), pp. 25–35.) A Z-SPIN method and others are also proposed. See, e.g., Japanese Patent Application, Publication No. 2002-289494 and PCT International Patent Application No. 03/021352. In these correction methods, a mask having a measurement pattern (hereinafter called “a focus monitor mask) is used that differs from the one used for an actual device mask (a mask used when actually manufacturing a semiconductor device and the like). Measuring a position-shifting of a pattern finds the best focus position and field curvature. For a step-and-scan exposure apparatus (i.e., a scanner) that requires a shorter inspection time and higher correction accuracy because it has many measuring points, these correction methods have the characteristics of monitoring focus changes during scanning, and the like. Accordingly, it is effective to use them in adjusting the position of an image plane (scan field curvature) related to a lens' field curvature of an apparatus and a change in posture during scanning.
A mask flatness measuring means uses a measuring optical system, and requires that the measuring optical system's original point be corrected. For correction of the original point, a flat plate that assures absolute flatness is typically used as a base, but in scanning exposure, the measuring optical system varies because it is in driving motion, and the original point also varies. Because of such variation errors and other errors, a measurement result obtained by using a focus monitor mask (e.g., a scan field curvature) and the one obtained by using an actual device mask do not necessarily agree. Further, due to a requirement for more minute devices, etc. in recent years, the impact of the errors on the imaging performance has become non-negligible.